In various types of signal and control systems for railway and mass and/or rapid transit operations, it is common practice to regulate or control the speed of the moving transit vehicles by modifying the tractive effort exerted by each vehicle of the train. The electrical traction motors on the vehicles are controlled over train lines which extend along the length of the transit vehicles. The traction motors are operated in series, parallel or combinations thereof to vary the tractive effort and in turn the speed of the moving train. The train lines carry various electrical signals to modify or establish the mode of operation of the traction motors on each of the vehicles in order to maintain the speed of the train as selected by the operator or engineer. These electrical signals may take the form of d.c. voltages which have discrete levels or amplitudes each of which signifies the particular mode at which the vehicle motors are to be operated to attain or sustain a given speed. Thus, it is desirable to provide regulated d.c. voltage signals at various discrete levels to obtain the optimum results and the most effective method of controlling operating conditions of the tractive motors. However, prior types of electronic regulators and regulating circuits are not wholly acceptable for usage in providing the necessary discrete d.c. voltages in train speed control equipment due to the presence of high voltage spikes and transients that occur on the train lines. In a conventional solid-state series or parallel regulating circuits, the semiconductive devices or transistors are susceptible to damage and destruction by the appearance of voltage surges or transients on the output end. That is, the output terminals and in turn the semiconductive devices of the previous regulating circuits are directly exposed to the voltage spikes and transients appearing on the train lines so that burned out components and circuit failures are common occurrences. The use of surge protection devices and circuits are not feasible in light weight rapid transit operations due to the additional weight and space requirement and are uneconomical due to the increased costs and maintenance expenses. Thus, there is an essential need for an improved solid-state regulator for use in vehicle speed control equipment which is not susceptible to damage or destruction by voltage surges or transients that may appear on the train lines of railway and mass and/or rapid transit vehicles.